Cements are materials which set and harden after combination with water as a result of chemical reactions with the water. After hardening, cements retain strength and stability even upon exposure to water. Most construction cements today are hydraulic cements, and most of these are based upon Portland cement, which is made primarily from limestone, certain clay minerals, and gypsum in a high temperature process that drives off carbon dioxide and chemically combines the primary ingredients into new compounds.
Setting and hardening of hydraulic cements is caused by the formation of water-containing compounds as a result of reactions between cement components and water. The reaction and the reaction products are referred to as hydration, hydrates, or hydrate phases, respectively. As a result of the immediately starting reactions, a stiffening (loss of workability) can be observed which is very small in the beginning, but which increases with time. After reaching a certain level, this point in time is referred to as the start of setting. The consecutive further consolidation is called setting (solidification), after which the phase of hardening (strength gain) begins. The compressive strength of the material then grows steadily, over a period which ranges from a few days in the case of “ultra-rapid-hardening” cements, to several years in the case of ordinary cements.
Set retarders are used to prevent premature hardening of the slurry before it reaches the area to be cemented. Set retarders prolong the setting time of the cement to allow time for the cement to be pumped into place.